Turn off uninitialized-use warnings for gcc in cmake build

Added support to the cmake build to turn off uninitialized use warnings
for gcc. This cleans the build up somewhat.

Used logic simpler than found in autoconf by making use of the fact that
although gcc won't complain about unsupported -Wno-* flags it *will*
complain about unsupported -W flags.

Reviewers: gribozavr, doug.gregor, chandlerc


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174299 91177308-0d34-0410-b5e6-96231b3b80d8